1. Field of the Invention
The present invention relates to a method for manufacturing a surface acoustic wave device that is preferably used as a surface acoustic wave resonator or a surface acoustic wave filter which utilizes a surface acoustic wave, such as Rayleigh wave, Love wave, or Leaky wave, and a surface acoustic wave device. More particularly, the present invention relates to a method, in which a step of forming an air gap in a portion in which a surface acoustic wave is excited is improved, a method for manufacturing a surface acoustic wave device, and a surface acoustic wave device.
2. Description of the Related Art
In the related art, the miniaturization of mobile communication equipment, such as mobile phones, has been in high demand. For this reason, the miniaturization of band-pass filters used in this type of communication equipment has been in high demand.
As compact band-pass filters, in the related art, surface acoustic wave filters that utilize surface acoustic waves have been commonly used. In Japanese Unexamined Patent Application Publication No. 7-115343, a surface acoustic wave filter device capable of being miniaturized is disclosed.
As shown in FIG. 7, in a method for manufacturing a surface acoustic wave device 101 that is disclosed in Japanese Unexamined Patent Application Publication No. 7-115343, first, a piezoelectric substrate 102 is prepared. On the top surface of the piezoelectric substrate 102, electrodes, such as IDT electrodes 103, are arranged to define a surface acoustic wave filter. The IDT electrodes 103 are provided in a portion of the piezoelectric substrate. The portion of the piezoelectric substrate is excited when the surface acoustic wave filter is used, and a surface acoustic wave is propagated. Accordingly, it is necessary to form an air gap A that the IDT electrodes 103 face. For this reason, as shown in FIG. 7, a cover member 104 is bonded to the piezoelectric substrate 102 so that the cover member 104 covers the top surface of the piezoelectric substrate 102 and so that the air gap A is maintained.
In the manufacturing method disclosed in Japanese Unexamined Patent Application Publication No. 7-115343, the bonding of the cover member 104 is performed by direct bonding without using an adhesive. Direct bonding is a method in which heat is applied to the piezoelectric substrate 102 and the cover member 104 with the piezoelectric substrate 102 being in contact with the cover member 104 to make hydrogen bonds therebetween, thereby directly bonding the piezoelectric substrate 102 and the cover member 104 together. Accordingly, the surfaces of portions facing the surfaces of the piezoelectric substrate 102 and the cover member 104 which are to be bonded must include hydrophilic groups. The hydrophilic groups are to be bonded by hydrogen bonds of water molecules provided from moisture.
Because the bonding is performed by direct bonding, no adhesive is required. A sealing property of portions of the cover member 104 and the piezoelectric substrate 102 to be bonded can be increased.
Because, in the surface acoustic wave filter device 101 disclosed in Japanese Unexamined Patent Application Publication No. 7-115343, it is necessary to form the air gap A, the size of the entire package is increased.
Additionally, a recessed portion used to form the air gap A must be provided in the cover member 104, or the cover member 104 having a recessed portion must be provided. Accordingly, there is also a problem that the cost is high.